Speed controller for hydraulic motors



Dec. 14, 1937. DQUGLAS 2,102,177

'SPEED CONTROLLER FOR HYDRAULIC MOTORS Filed Dec. 24, 1934 4 Sheets-Sheet l INVENTUR '17 Mon; 2L K-DnuBgA's ATTURNEY.

Dec. 14,'1937. J K D S 2,102,177

SPEED CONTROLLER OR HYDRAULIC MOTORS I Filed Dec. 24, 1934 4 Sheets-Sheet 2 32 3/ R 4 i 30: a 4 A! I v, I

/9 g. .2 c E B B7 BIO C8 ,2 q B Flig- 25 ATTEIRNEY.

" Dec. 14, 1937.

' J. "K. DOUGLAS 2,102,177

SPEED CONTROLLER FOR HYDRAULIC MOTORS Filed Dec. 24, 1954 4 Sheets-Sheet 3 m k m QR m D D U 4 f;

JAMES K-DEIUGLAS TITEIRJNEY.

INVENTEIR' Patented Dec. 14, 1937 UNITED STATES SPEED CONTROLLER FOR HYDRAULIC MOTORS James K. Douglas, Milwaukee, Wis., assignor to The'Oilgear Company, Milwaukee, Wis., a corporation of Wisconsin Application December 24, 1934, Serial No. 758,966

14 Claims.

This invention relates to a hydraulic transmission in which the motor is driven at a predetermined'speed whenever the pump is operating at a constant speed and constant displacement. More particularly; it relates to an apparatus for maintaining the motor speed constant, such as the apparatus disclosed in Patent No. 2,004,522 issued June 11, 1935 on an application filed September 19, 1932, Serial No. 633,750, of which this application is a continuation in part.

The pump has as an inherent characteristic thereof an internal slip or leakage which reduces its net delivery. This slip is made up of several small factors, such as the small volumes which escape from between the cylinders and the pump valve, the small volumes which pass across the face of the pump valve from the discharge port to the intake port thereof, and any small volumes which may leak past the pistons. In addition, the

net delivery of the pump is decreased by an apparent slip due to deflection of its parts, expansion of the pipes into which it delivers its output and to the compressibility of the driving liquid, which is ordinarily a good grade of lubricating oil.

If the motor is of the rotary type, it has a. slip composed of substantially the same factors which compose the slip of the pump. If the motor is of the reciprocating type, its slip is due principally to any liquid which may escape past the motor piston.

Due to the compressibility of' the liquid and to the fact that liquid will escape at an increased rate in response to an increase in pressure, the slip of both the pump and the motor at any given temperature will increase as the pump pressure increases and decrease as the pump pressure decreases. Consequently, the speed of the motor will decrease as the pressure increases and increase as the pressure decreases unless provision is made to compensate for variations in the combined slip of the pump and the motor due to variations in pressure.

The slip of both the pump and the motor is also affected by variations in temperature due to the fact that the fluidity of the oil ordinarily employed as a'driving medium increases as the temperature increases and decreases as the temperature decreases and to the fact that certain coacting pump parts ordinarily have different coefiicients of expansion so that an increase in temperature will cause'one part to expand more rapid- 1y than another part and thereby vary the clearance therebetween. Consequently, the combined slip of the pump and the motor will vary in ac (CI. 60-52) g cordance with variations in temperature and cause the speed of the motor at any given pressure to decrease as the temperature of the oil increases and to increase as the temperature of the to variations in the temperature of the oil.

The above patent discloses an apparatus which will compensate for variations in slip due to variations in pressure and an apparatus which will compensate for variations in slip due to variations in both temperature and pressure. Each apparatus is provided with a plurality of control elements each of which is individually adjustable and, in order that the apparatus may be preset or adjusted at the factory, certain data are necessary such as the character of the oil to be employed, the temperaturelimit, the maximum working pressure and the amount of slip.

Thepresentinventionhasas anobject to provide an apparatus which is an improvement over the patented apparatus.

Another object is to provide a speed controlling apparatus in which a plurality of control elements are adjustable simultaneously.

Another object is to provide a speed controlling apparatus which may be adjusted without previous knowledge of all conditions under which it is to operate.

Another object is to provide a speed controlling apparatus which is susceptible of close adjustment and control.

Another object is to provide a speed controlling apparatus which is positive and precise in operation.

Other objects and advantages will appear from the following description of the apparatus illustrated in the accompanying drawings which are somewhat diagrammatic in character and in which the views are as follows:

Fig. 1 is atop plan view of a slip compensator in which the invention is embodied.

Fig. 2 is a sectional plan view of the slip compensator, the section being taken substantially along the line 2-2 shown extending through Figs. 3, 4 and 5. l

Fig. 3 is a vertical section taken on the line 3-3 Fig. 7 is a vertical section taken on the line 1-4 Of Fig: 1.

Fig. 8 is a vertical section taken on the line 88 of Fig. 1.

Fig. 9 is a vertical section taken on the line 9--9 of Fig. 2 with certain parts shown in elevation. i

Fig. 10 is a-sectional plan view taken on the line Ill-l0 of Fig. 9 but shown turned at an angle to the disclosure of Figs. 2 and 9.

Fig, 11 is in part a diagram of a hydraulic circuit containing a hydraulic motor the speed of which is to be controlled and in part a schematic drawing showing the relation of the several parts of the slip compensator to each other and to the hydraulic circuit.

The slip compensator, as shown, consists primarily of a pressure inverter A, a resistance B consisting of a choke and an orifice connected in series with each,other and connected to the pressure inverter A, a constant pressure valve C connected in parallel with the pressure inverter A, a temperature responsive resistance valve D connected in series with the valve C, and a throttle E connected in series with the temperature responsive resistance valve D, all of which are arranged in a casing ll having an inlet l2 (Fig. 3) and an outlet [3 (Figs. 1 and 8).

For the purpose of illustration, the slip compensator is shown in .Fig. 11 as being connected to a hydraulic transmission consisting of a pump [4 and a motor l5 which are connected to each other by two pipes l6 and I! having a reversing valve l8 connected therein. Liquid is discharged by the pump into the pipe l6 and liquid is returned from the motor to the pump through the pipe H. The slip compensator has its inlet l2 connected to the pipe l6 by a pipe l9 and its outlet l3 connected to the pipe I! by a pipe 20.

The pressure inverter A (Figs. 3 and 6) is arranged in a vertical bore 2| which is formed in the casing l I and communicates at its lower end through an annular valve seat 22 with the inlet l2.

The lower end of the bore 2|. is connected by a passage 23 (Fig. 6) to the lower end of a vertical bore 24 which is formed in the casing II and contains the resistance B.

The bore- 24 is closed at its lower end by a plug 25 and connected intermediate its ends by a passage 26 (Fig. .7) to a vertical bore 21 which is formed in the casing II and contains stant pressure valve C."

The bore 21 is closed at' its lower end by a cap 28 fastened to the bottom of thecasing II, and the bores 2|, 24 and 21 are closed at the upper ends thereof by a cap 23 fastened to the topof the casing ll.

Liquid entering the inlet l2 may flow through the pressure inverter A and thepassage 23 into the resistance B and be discharged fromthe resistance B into the passage 26.

The bore 21, at a point opposite the passage 26, is connected by a passage 30 (Fig. 8) to the lower end of a vertical bore 3| which contains the throttle E and is open to the outlet l3 at a point opposite the passage 30. The passage 26, the

.bore 21, the passage 30, and the bore 3| provide a channel through whichliquid exhausted from the resistance B may escape freely through the outlet I3.

The inlet I2 is connected to the bore 21 by a passage 32 (Fig. 3) which extends horizontally from the inlet, then vertically upward and then horizontally and enters the bore 2'! at a point a short distance below the upper end thereof,

the con- The upper end of the bore 21 is open to a passage 33 (Fig. 4) which is formed in the cap 29 and communicates with a vertical passage 34 formed in the casing I 1. The lower end of the passage 34 communicates with a horizontal passage 35 which has one of its ends open to the bore 21 at the lower end thereof and its other end open to a vertical bore 36 formed in the lower part of the casing II.

The bore 36 contains a part of the temperature responsive resistance valve D which controls communication between the passage 35 and a chamber 31 (Figs. 5 and 8) which is formed in the casing ll and closed by a cap plate 38 fastened to the end of the casing.

The chamber 31 communicates with the bore 3| through a smaller bore 39 which is concentric with the bore 3| and has a part of the throttle E fitted therein.

Liquid may flow from the inlet l2 through the passage 32, the constant pressure valve C, the passages 34 and 35, the bore 36, the temperature responsive resistance valve D, the chamber 31 and the throttle E to the outlet I3. The valve C keeps the pressure of the liquid entering the valve D substantially constant, the valve D responds to variations in temperature to vary the volume which may flow therethrough, and the throttle E restricts the flow of liquid into the outlet l3.

Pressure inverter A As best shown in Figs. 3 and 6, the bore 21 has fitted in the upper part thereof a hollow piston A which has a valve stem A extending through its lower end wall or head and forming an oiltight joint therewith. The stem A extends through the valve seat 22 and carries a valve A upon its lower end to coact with the seat 22 and thereby control the flow of liquid from the inlet l2 to the passage 23.

The piston A is urged downward by a helical compression spring A which is arranged inside the same and has its tension adjusted by a screw A threaded through the cap 29. In order that the spring may not tend to tilt the piston in the bore, it is preferably arranged between two springretainers which are pivoted, respectively, upon the ends of the stem A and the screw A as shown in Figs. 3 and 6.

The downward movement of the piston A is limited by an annular shoulder A which is formed in the wall of the bore 2i and against which the spring A tends to hold the piston A to thereby tend to maintain the opening between the valve A and the valve seat 22 at a predetermined maximum.

As previously explained, the inlet I2 is connected to the discharge side of the pump I4 and the outlet l 3 is connectedto the intake side of the pump. Consequently, the slip compensator permits liquid to be bypassed at limited rates across the two sides of the hydraulic circuit and pump pressure prevails at the inlet l2.

The flow of liquid through the pressure inverter is retarded initially by the resistance B which causes pressure to build up in the lower end of the bore 2| and act upon thelower end of the piston A The force exerted upon the piston A by the liquid in the lower end of the bore 2| and the force exerted upon the lower face of the valve A by the pressure at theinlet tend to move the piston A upward against the resistance of the spring A In order that movement of the piston A may not behampered by entrapped air or liquid, the

' enough force, at maximum pump pressure, to

openingbetween the valve A and the valve seat 22, thereby throttling the flow of liquid into the bore 2i and causing a pressure differential betweenthe inlet l2 and the lower end of the bore As the pump pressure increases, the force ex erted by the liquid upon the valve A .will increase in proportion and move the valve A nearer the seat 22, thereby increasing the throttling eifect and causing the pressure in the lower end of the bore 26 to decrease as pump pressure increases. Consequently, the pressure of the liquid delivered to the resistance B will vary inversely in proportion to variations in pump pressure.

The area of the valve A is not great enough to permit the liquid at the inlet. to exert a great compress the spring A enough to permit the valve A to seat firmly upon the seat 22. In other words, the force exerted upon the valve A by the high pressure liquid at the inlet must be augmented by a force exerted upon the lower face of the piston A by the low pressure liquid in the lower end of the bore 2!! in order to overcome the resistance of the spring A If a sudden rush of pressure should cause the valve A to close, the pressure in the lower end of the bore 2i would drop and permit the valve to immediately reopen.

Resistance B,

The resistance B consists of a choke and an orifice which are shown connected in series with each other for the reason that this arrangement produces the desired result when the slip com-'- pensator is connected to a pump of the type ordinarily employed in installations in which it is desired to drive the machine or a part thereof at a constant speed during a given period of time.

The slip of this type of pump does not vary wholly in accordance with the law of viscous flow nor wholly in accordance with the law of turbulent fiow but varies at a rate which may be readily obtained by'connecting a choke and anorifice in series.

However, it is to be understood that the resistance B may consist of either a choke, an orifice, a choke and an orifice in parallel, or a choke and an orifice in series, depending upon the characteristics of the slip of the transmission to which the slip compensator is to be connected. I

Asshown in Figs. 2, 6, 7, 9, and 10, the resistance 18 is provided with a cylindrical sleeve B which is tightly fitted in-the bore 2%. The sleeve B is provided in its lower part with a bore 15 and in its upper part with a counterbore 13 which forms an annular shoulder 15 in the wallof the sleeve B at its junction with the bore B The sleeve 3* extends from a point near the top of the bore 2tto a point just above the entrance to the passage 23 (Fig. 6) so that liquid passing through the passage 23 fromthe inverter A is directed into the bore B which has a hollow cylindrical member B closely 'flttedtherein with its lower end open to the passage 2.3.

The member B has a slot B (Figs. 7 and formed in its side wall and partly in registry with an elongated slot B which is formed in the sleeve B and extends from a point just above the bottom of the sleeve B to a point a substantial distance above the shoulder B as shown in Fig. 6.

The wall of the member B ordinarily covers a part of the slot B so that the edges of the slots B and B define a restricted passage or orifice B (Figs. 7 and 10) through which liquid discharged from the passage 23 must pass. The cross-sectional area of the orifice B may be varied by rotating the member B to vary the wic l th of the orifice or by moving the member B vertically to vary the length of the orifice.

The member B is supported by a bolt B which extends through the upper wall thereof and is threaded into a cylindrical choke member 13 arranged in the upper part of the sleeve B The member B is normally fixed for axial movement with the choke B by the bolt B which may be loosened to permit the member B to be rotated to vary the width of the orifice B The choke member B has its upper part closely fitted in the counterbore B and its lower part reduced in diameter and closely fitted in the bore 'B an annular shoulder B being formed upon the periphery of the member B between the upper and the lower parts thereof.

The shoulder B is spaced above the shoulder B to provide between the periphery of the choke member B and the wall of the counterbore B a restricted passage B which functions as a choke. The choke passage B is open at one point to the slot B in the sleeve B (Fig. 6) and at another point it communicates with an aperture B (Fig. 7) which is formed in the wall of the sleeve B and registers with the passage 26.

Choke member B is supported by a stem B which is threaded therein and extends through cap 29 and through a gland ti fastened to cap 29'. Stem B is provided with means for rotating it, as by having its upper end slotted, and it is restrained from axial movement as by having two collars fixed thereon and in contact, respectively, with the lower face of cap 29 and the upper face of gland M.

Rotation of the stem will raise or lower the choke member El and vary the distance between the shoulders B and 3, thereby varying the cross-sectional area of the passage B to vary the volume of liquid which can flow ther'ethrough at any given pressure.

Rotation of the stem B will also raise or lower the cylindrical member B and thereby vary the cross-sectional area of the orifice B by Varying the length thereof.

The choke and the orifice may thus be simultaneously adjusted by simply rotating the stem 3. Also, the relation between the choke and. the orifice may be varied by rotating the cylindrical member 13 as previously explained.

The choke member B is prevented from rotating by a key B (Figs. 2 and 9) fixed therein and closely fitted for vertical movementin a complementary keyway which is formed in the wall of the sleeve B and has suflicient length to permit the member B to move vertically a substantial distance.

The slot 13'' and theap'erture B hare formed close together in the sleeve B at one side of the choke member B and the key B is arranged Thus, as viewed in Fig. 2, the choke passage B extends from the left edge of the slot B around the member B to the right edge of the aperture B Its length is fixed but its crosssectional area may be varied by varying the distance between the shoulders 13* and previously explained.

Liquid entering the bore 24 from the pressure inverter A flows through the orifice B (Figs. '7 and 10) at a relatively high speed and then flows upward at a reduced speed through the slot 13'' (Fig. 6) due to the cross-sectional area thereof being much greater than the area of the orifice 3*. From the slot B", the liquid flows through the choke passage B the aperture B (Fig. 7) the passage 26, the bore 21, the passage 30 (Fig. 8), the bore 3| and the outlet l3 to the return pipe 20.

Constant pressure valve C e The bore 21 (Figs. '2, 3 and 4) has a plunger C fitted in the lower part thereof to reciprocate therein and a plug C rigidly secured in the upper part thereof and provided with an axial bore C and with a plurality of radial ducts C which intersect the bore C at a point intermediate the ends thereof.

The outer ends of the ducts C open into an annular groove C which is formed in the periphery of the plug C and registers with the upper end of the passage 32 (Fig. 3) so that liquid may flow from the inlet I2 into the bore C Communication between the bore C f, and the ducts C is controlled by a valve C which is fitted in the bore C and has its head supported by the reduced upper end of the plunger C so that raising the plunger C a greater or lesserdistance will cause the flow of liquid into the bore C to be cut off or restricted. The bore C ordinarily has suitable bushings fixed therein above and below the intersection with the ductsC to provide a bearing for the valve 0".

The plunger C and the valve C are urged downward by a helical compression spring C arrangedin the bore 21 between two spring retainers (3 and C which bear, respectively, against the reduced lower end of the plug C and upon the head of the valve C in order to retain the head of the valve C in contact with the plunger C and to prevent thespring from bearing unevenly upon the plunger C and thereby tending to tilt it in the bore 21, s

As previously explained, liquid discharged from.

positive pressure due to the f'actthat the pump I4 ordinarily has connected therewith an auxiliary pump (not shown) which delivers liquid into the intake side of the ump at a constant low pressure to compensate for leakage losses and it thereby maintains a constant low pressure in the I pipes I1 and 20 and in the passages 26 and 30.

Whenthe plunger C is in its lower position, liquid may flow freely from the passage 32 through the ducts C the bore C and the passages 33, 36 and 35 into the lower ends of the bores 21 and.

Discharge of liquid from the bore 36 is resisted by the temperature responsive resistance valve D, and this resistance causes pressure to be created in the lower ends of the bores 21 and 36. This pressure acts upon the lower end of the plunger C and urges it and the valve C upward with a force proportional to the diflerence between the pressure in the lower end of the bore 21 and'the pressure in the bore 21 above the plunger C Upward movement of the plunger C causes the valve C to throttle the flow of liquid from the ducts G into the bore C and thereby causes a drop in pressure therebetween.

As the pressure at the inlet l2 rises, the pressure in the lower end of the bores 21 and 36 will rise until the drop in pressure across the plunger C reaches a value corresponding to the resistance of the spring 0'' at which time the liquid in the lower end of the bore 21 has raised the plunger C until the valve C produces sufficient throttling effect to maintain the pressure in the lower ends of the bores 21 and 36 at the value determined by the resistance 'of the spring C". Therefore, when the slip compensator is functioning, liquid is being delivered to the temperature responsive resistance valve D at a substantially constant low pressure.

While the resistance of a spring increases as it is deflected, the distance through which the valve C is moved after the valve D opens is so short that the increase in the resistance of the spring C is negligible or substantially so and has little efiect on the pressure at which liquid is delivered to the valve D. If this very slight increase in the resistance of the spring C should cause an appreciable variation in the rate of flow Temperature responsive resistance value D The flow of liquid from the passage 35 (Fig. 4) to the outlet l3is retarded in the first instance by a ball valve D (Fig. 5) arranged within the chamber 31 upon an annular valve seat D which is threaded into the bore 36 and through which 'the passage 35 communicates with the chamber 31.

The ball valve D is urged against the seat D by a helical compression spring D arranged partly in the upper end of the bore 36 and partly in the chamber 31 between a screw plug D which closes the upper end of the bore'36, and a socket D which bears upon the ball valve D and through which the spring exerts a force thereon.

The action of the spring D upon the valve D is modified by a temperature responsive device such as a bimetallic strip D (Figs. 5 and 8) which is arranged in the chamber 31 and im- ,mersed in the liquid which flows past the valve D The strip D is -wound-in a spiral and has one 'of its ends in engagement with the socket I D and its other end fixed to a stationary pin D" fixed to the casing ll.

The function of the device D is to vary the spring pressure upon the valve D in response valve inrespons to an increase in temperature or it may. ofi'er'an initial resistance to the action of the spring D and then decrease thatresistance in response to an increase in tempera-- ture, thereby varying the flow or liquid past the ature of the liquid.

Throttle E The discharge of liquid from the chamber 31 to the outlet I3 is controlled by a valve E (Figs. and 8) which is closely fitted in the bore 39 and .fixed at its upper end to a screw E which is considerably larger in diameter, threaded into the bore 3| and provided with agland E to prevent leakage of liquid.

The valve E is provided in its lower end. with one or more tapered grooves E which, together with the Wall of the bore 39, form small orifices through which liquid may flow at limited rates from the chamber 31 to the outlet l3. The area of the orifices may be variedby turning the screw E to raise or lower the valve E The throttle E restricts the flow of liquid from the chamber 31' to the outlet l3 and thereby causes a higher pressure to prevail in the chamber 31 than prevails in the outlet I 3. The rate of fiow through the orifices E is a function of the difference in pressure upon opposite sides thereof.

When the temperature of the liquid is at a predetermined minimum, the valve D ofiers a predetermined minimumresistance to" the flow of liquid therethrough and thereby. permits a predetermined maximum pressure to be created in the chamber 3'! so that liquid flows at a predetermined maximum rate through the throttle E.

As the temperature of the liquid rises, the valve D ofiers increased resistance to the flow of liquid therethrough with a resultant drop in the pressure in the chamber 37. As the pressure in the chamber 31 decreases, the flow of liquid through the throttle E decreases, thereby varying the flow inversely to variations in the temperature of the liquid.

Operation Assuming that the slip compensator is connected to a hydraulic transmission 'as shown in Fig. 11, that the pump 14 is delivering liquid 'tothe motor through the pipe l6 and that the motor is returning liquid to the pump through the pipe ll, liquid will be bypassed at a limited volumetric rate from the pipe lfi'through the slip compensator to-thepipe I1.

This liquid will enter the inlet [2 at pump pressure, pass through the slip compensator and be returned to the pump at the constant low pressure prevailing in the return pipe'll.

The reason for directing the discharge from the slip compensator into the return pipe I! is that the combined slip of the pump and motor varies directly in'accordance with variations in the drop in pressure across the motor or between the discharge pipe l6 and the return pipe ll. It is for this reason that the upper surfaces of the plungers A and C are subjected to return pipe pressure and are each urged downward by a force proportional thereto.

If the motor discharged into a reservoir and the pump received liquid from the reservoir at atmospheric pressure, the slip compensator could discharge into the reservoir and it would not be necessary to subject the pistons A and C to the return pressure but simply to provide vents above the pistons A and C Therefore, the liquid pressure acting uponthe-upper surfaces of the pistons A and C has been disregarded in the explanation given herein.

The liquid to be bypassed flows from the pipe I 6 through the pipe l9 into the inletl2 at pump pressure and in a single stream as indicated by the arrow superimposetLupon the pipe l9 in Fig.

After passing through the inlet l2, the liquid is divided into two streams. One stream flows through the pressure inverter A and the resistance B, as indicated by left half-arrows, and has its rate of flow varied inversely to variations in pump pressure. The other stream flows through the constant pressure valve 0, the temperature responsive resistance valve D, and the throttle E, as indicated by the right half-arrows, and has its rate of flow varied inversely to variations in'its temperature. The two streams are united at the outlet l3 and returned to the pump at the return pressure and in a single stream, as indicated by the full arrows superimposed upon the pipe 20.

When the pump pressure rises above a prede-v termined minimum, the piston A is raised by the liquid pressure so that the valve A throttles the flow through the valve seat 22 to reduce the pressure in the lower end of the bore 2| in proportion to the rise in pump pressure above that minimum. If pump pressure fluctuates, the inverter A will vary the pressure inthe'lower end of the bore 2! and in the passage 23 inversely to variations in pump pressure as previously explained.

From the pressure inverter A, the liquid flows through the resistance? at a rate which varies according to variations in the pressure at which liquid is delivered thereto and, since this pressure varies inversely to pump pressure, the flow through the resistance B will vary inversely to pump pressure.

By adjusting the relation between the orifice B and the choke passage 13 to obtain a flow corresponding to the variation characteristics of the slip of the transmission and then turning the screw B -to adjust the total amount of orifice and choke elfect to correspond to the amount of slip of the transmission due to variations in pressure, the flow of liquid through the inverter A and the resistance B will be varied inversely at exactly the same rate at which the slip of the transmission varies in response to variations in pump pressure, thereby maintaining the speed of the motor l5 constant at any given temperature regardless of any and all changes in pump pressure.

In order that the speed of the motor l5 may be maintained constant throughout the entire range of operating pressures and temperatures, the flow through the slip compensator must also be varied inversely tovariations in the slip of the transmission due to variations in the temperature of the liquid. This is accomplished by the valves C and D and the throttle E which function as follows:

Before pump pressurereaches a predetermined minimum, it extends through the passage 32 (Figs. 3, 4 and 11) 'the ducts C the bore C and the passages 33, 34 and 35 into the lower ends of the bores 21 and 36.' The valve D resists the escape of liquid from the lower end of the bore 36 and enables the pressure to act upon the lower face of the plunger C When pump pressure rises above the predetermined minimum, the forceexerted by the liquid upon the plunger C is suflicient to raise it and the valve C against the resistance of the spring C The valve C throttles the flow from the ducts C to the bore C and causes a drop in pressure therebetween.

As the pump pressure continues to rise, the

force exerted by the liquidupon the plunger C increases and the valve continues to rise and increase its throttling effect until it is permitting passage of only suflicient liquid to maintain a predetermined low pressure in the lower ends of the bores 2! and 36. The valve C thus functions as a constant pressure valve and causes liquid to be continuously delivered to the valve D at a substantially constant low pressure after pump pressure exceeds a predetermined minimum.

From the passage 35, the liquid flows through the valve D and the throttle E to the outlet I3. The valve D causes a drop in pressure between the passage 35' and the chamber 31, and the throttle E causes a drop in pressure between the chamber 31 and the outlet l3.

The fiow of liquid through the throttle E follows the law of fiow through an orifice. That is, the flow therethrough varies in accordance with variations in the drop in pressure across the orifices E and is but little affected by a change in the temperature of the oil.

The drop in pressure across the valve D is proportional to the combined resistance of the spring D and the bimetallic strip D The force exerted by the spring D upon the ball valve D remains constant but the strip D responds to variations in the temperature of the oil to increase this force as the temperature rises and to decrease it as the temperature decreases. F

Therefore, since the liquid pressure in the passage 35 is maintained substantially constant by the valve C, the valve D will vary the pressure in the chamber 31 inversely to variations in the temperature of the liquid.

Since the fiow of liquid through the throttle E is varied in accordance with the drop in pressure across the same, and since the pressure remains constant in the outlet l3 and the pressure in the chamber 31 varies inversely to variations in the temperature of' the liquid, the flow through the throttle E will vary inversely to variations in the temperature of the liquid. i

Therefore, since the liquid is delivered to the valve D at a substantially constant pressure, the flow therethrough and through the throttle is not aflected by variations in pressure but varies solely in response to variations in temperature. Consequently, the valves C and D and the throttle E operate to vary the rate of fiow of the bypassed liquid inversely to variations in the slip of the transmission due to variations in temperature. thereby operating to maintain the speed of the motor l constant at any given temperature.

The slip of a hydraulic transmission is greatest when the pressure and the temperature of the liquid are the highest and is the least when the pressure and the temperature of the liquid are the lowest. If a transmission could operate at zero pressure, its slip would be zero at zero pressure but it cannot operate at a pressure less than the minimum pressure required to overcome the inertia and friction ,of its moving parts.

The slip compensator is initially so adjusted that, at the minimum pump pressure and at the lowest temperature under which the transmission is to operate, liquid will be bypassed therethrough at a rate at least as great as the rate of slip when the pressure and temperature of the liquid, are at the highest points which will be encountered. Therefore, the pump pistons must move, during a given period of time, a volume-of liquid equal to the consumption of the motor, the slip of the transmission and the volume bypassed during that period of time.

It is sometimes desirable to have a small change in speed in response to a variation in the temperature or the pressure of the liquid. This may be accomplished, within the limits of the compensator, by simply adjusting the apparatus to under compensate or to over compensate as desired.

The invention herein set forth is susceptible of various modifications and adaptations without departing from the scope thereof as hereafter claimed.

The invention is hereby claimed as follows:

1. The combination, with a hydraulicmotor, a pump normally delivering motive liquid at a rate in excess of the rate required to drive said motor at a predetermined speed, a pressure channel for directing motive liquid from said pump to said motor and a return channel for directing exhaust liquid from said pump to said motor, of a slip compensator connected between said pressure channel and said return channel and comprising means for subtracting liquid from the output of said pump at a limited rate and responsive to variations in pump pressure to vary said limited rate inversely to said variations in pressure, and

a second means connected in parallel with the aforesaid means for subtracting liquid from the output of said pump at a limited volumetric rate and responsive to variations in the temperature of the liquid delivered by said pump to vary said volumetric rate inversely to said variations in temperature. I

2. The combination, with a hydraulic motor and a pump for supplying liquid to said motor to drive the same and normally delivering liquid at a rate in excess of the rate required to drive said motor at a predetermined speed, of a slip compensator comprising a viscous flow choke and an orifice choke connected in series with each other and with the outlet of said pump and together forming throttling means for subtracting liquid from the output of said pump at limited rates, a pressure inverter connected between said pump and said throttling means for varying the pressure at the inlet to 'said throttling means inversely to variations in pump pressure, and means for adjusting said choke and said orifice simultaneously.

3. The combination, with a hydraulic motor and a pump for supplying liquid to said motor to drive the same and normally delivering liquid at a rate in excess of the rate required to drive said varying the relation between said choke and said 4. The combination, with a hydraulic motor and a pump for supplying liquid to said motor to drive the same and normally delivering liquid at a rate in excess of the rate required to drive said motor at a predetermined speed, of a slip compensator comprising a choke and an orifice connected in series with each other and with the outlet of said pump and forming throttling means i for. subtracting liquid from the output of said pump at limited rates, a pressure inverter connected between said pump and said throttling means for varying the pressure at the inlet to said throttling means inversely to variations in pump pressure, means connected in parallel with.

said pressure inverter for subtracting liquid at a limited rate from the liquid delivered by said pump, and temperature responsive means for varying said limited rate inversely to variations in the temperature of said liquid.

5. The combination, with a hydraulic motor and a pump for supplying liquid to said motor to ,drive the same and normally delivering liquid at a rate in excess of the rate required to drive said motor at a predetermined speed, said'pump and motor having a combined slip which varies in accordance with variations in the pressure and the temperature of said liquid, of a slip compensator connected to said pump for subtracting liquid from the output thereof at a limited volumetric rate and having means responsive to variations in pressure to vary said limited .rate inversely to said variations in pressure and means responsive to variations in temperature to vary said limited rate inversely to said variations in temperature; said temperature responsive means including a resistance valve having means for varying the resistance thereof in response to variations in the temperature of the liquid delivered thereto, a throttle connected to the outlet of said resistance valve for retarding the discharge of liquid therefrom, and a constant pressure valve connected between said pump and the inlet of said resistance valve and operable to deliver liquid to said resistance valve at a substantially constant pressure after pump pressure exceeds a predetermined minimum.

6. The combination, with a hydraulic motor and a pumpfor supplying liquid to said motor to drive the same and normally delivering liquid at a rate in excess of the rate required to drive said motor at a predetermined speed, of a slip compensator comprising means for subtracting liquid from the output of said pump at a limited rate and responsive to variations in pump pressure to vary said limitefiate inversely to said variations in pressure, and\a second means for subtracting liquid from the output of said pump at a limited volumetric rate and responsive to variations in the temperature of the liquid delivered by said pump to vary said volumetric rate inversely to said variations in temperature; said pressure responsive means including a resistance having a choke and an orifice connected in series, and a pressure inverter connected between said pump and said resistance for varying the pressure of the liquid delivered to said resistance .inversely to variations in pump pressure.

liquid from the output of said pump at a limited rate and responsive to variations in pump pressure to vary said limited rate inversely to said variations in pressure, and a second means for subtracting liquid fromthe output of said pump at a limited volumetric rate and responsive to variations in the temperature of the liquid delivered by said pump to vary said volumetric rate inversely to said variations in temperature; said pressure responsive means including a resistance having a choke and an orifice connected in series and means for adjusting said choke and said orifice simultaneously, and a pressure inverter connected between said pump and said resistance for varying the pressure of the liquid delivered to said resistance inversely to variations in pump pressure.

8. In a slip compensator for connection to a pumpv to subtract liquid therefrom at a limited volumetric rate and thereby compensate for variations in the slip of saidpump, the combination of a resistance valve having means for varying the resistance thereof in response to variations in the temperature of the liquid delivered thereto, a throfile connected to the outlet of said resistance valve for retarding the discharge of liquid therefrom, and a constant pressure valve connected between said pump and the inlet of said resistance valve and operable to deliver liquid to said resistance valve at a substantially constant pressure after pump pressure exceeds a predetermined minimum.

9. In a slip compensator for connection to a pump to subtract liquid therefrom at a limited volumetric rate and thereby compensate for variations in the slip of said pump, the combination of a resistance valve having a spring urging it to closed position and a thermal element coacting with said spring and responsive to variations in the temperature of said liquid to vary the efiect of said spring and thereby vary the flow of liquid through said valve in response to variations in the temperature of said liquid, a throttle connected to the outlet of said resistance valve for retarding the discharge of liquidtherefrom, and a constant pressure valve connected between said pump and the inlet of said resistance valve and operable to deliver liquid to said resistance valve at a substantially constant pressure after pump pressure exceeds a predetermined minimum.

10. A slip compensator, for connection to a pump to subtract liquid therefrom at a limited volumetric rate and thereby compensate for variations in the slip of said pump, comprising a choke and an orifice connected in series, a pressure inverter connected to said pump and in series with said choke and said orifice and responsive to variations in pump pressure for causing the pressure of the liquid passing through said choke and said orifice to varyinversely to variations in pumppressure, a resistance valve connected to said pump, a spring urging said resistance valve toward closed position, a thermal element coacting with said spring and responsive to variations in the temperature of the liquid for varying the efiect of said spring upon said valve,

a throttle connected to the outlet of said resistance valve for retardingthe discharge of liquid therefrom, and a constant pressure',valve connected between said pump and the inlet of said resistance valve for limiting the pressure of the liquid delivered to said resistance valve and operable to maintain substantially constant the pressure of the liquid delivered to said resistance '7 valve after pump pressure exceeds a predetermined minimum.

11. A slip compensator, for connection to a pump to subtract liquid therefrom at a limited volumetric rate and thereby compensate for variations in the slip of said pump, comprising a choke and an orifice connected in "series, means for adjusting said choke and said orifice simultaneously, a pressure inverter connected to said pump and in series with said choke-and said orifice and responsive to variations in pump pressure for causing the pressure of the liquid passing through said choke and said orifice to vary inversely to variations in pump pressure, a resistance valve connected to said pump, a spring urging said resistance valve toward closed position, a thermal element coacting with said spring and responsive to variations in the temperature of theliquid for varying the effect of said spring upon said valve, a throttle connected to the outlet of said resistance valve for retarding the dis and operable to maintain substantially constant the pressure of the liquid delivered to said resistance valve after pump pressure exceeds a predetermined minimum.

12. A slip compensator, for connection to a pump to subtract liquid therefrom at a limited volumetric rate and thereby compensate for variations in the slip of said pump, comprising a choke and an orifice connected in series, means for adjusting the relation between said choke and said orifice, a pressure inverter connected to said pump and in series with said choke and said orifice and responsive to variations in pump pressure for causing the pressure of the liquid passing through said choke and said orifice to vary inversely to variations in pump pressure, a resistance valve connected to said pump, a spring urging said resistance valve toward closed position, a thermal element coacting with said spring and responsive to variations in the temperature of the liquid for varying the effect of said spring upon said valve,

a throttle connected to the outlet of said resistance valve for retarding the discharge of liquid therefrom, and a constant pressure valve connected between said pump and the inlet of said resistance valve for limiting the pressure of the liquid delivered to said resistance valve and oper-' able to maintain substantially constant the pressure of the liquid delivered to said resistance valve after pump pressure exceeds a predetermined minimum.

13. A slip compensator, for connection to a pump to subtract liquid therefrom at a limited volumetric rate and thereby compensate for variations in the slip of said pump, comprising a choke and an orifice connected in series, means for adjusting said choke and said orifice simultaneously, means for adjusting the relation between said choke and said orifice, a pressure inverter connected to said pump and in series with said choke and said orifice and responsive to variations in pump pressure for causing the pressure of the liquid passing through said choke and said orifice -to vary inversely to variations in pump pressure,

a resistance valve connected to said pump, a spring urging said resistance valve toward closed position, a thermal element coacting with said spring and responsive to variations in the temperature of the liquid for varying the eifect of said spring upon said valve, a throttle connected to the outlet of said resistance valve for retarding the discharge of liquid therefrom, and a constant pressure valve connected between said pump and the inlet of said resistance valve for limiting the pressure of the liquid delivered to said resistance valve and operable to maintain substantially constant the pressure of the liquid delivered to said resistance valve after pump pressure exceeds a predetermined minimum.

14. The combination, with a hydraulic motor and a pump for supplying liquid to said motor to drive the same and normally delivering liquid at a rate in excessof the rate required to drive said motor at a predetermined speed, of a slip compensator comprising a resistance connected to the outlet of said pump and including a choke and an orifice forming throttling means for subtracting liquid from the output of said pump at limited rates, a pressure inverter connected between said pump and said resistance for varying the pressure at the inlet to said resistance inversely to varia- JAWS K. DOUGLAS. 

